Innermost stable circular orbits around rotating quark stars and kHz QPOs in low mass X-ray binaries

It has been suggested that observations of quasi periodic oscillations (QPOs) in the X-ray fluxes of low mass X-ray binaries can be used to constrain the masses of the compact members of these binaries and their equations of state. A basic assumption of the present paper is that the frequency of upper QPO is related to the Kepler frequency at the innermost stable circular orbit of the accrection disk around a strange star. Here, a summary of our studies of quark star masses and rotational and orbital frequencies is presented. The results are based on relativistic calculations of uniformly rotating strange star models described by the MIT bag model and the recently proposed QCD-based equation of state of strange quark matter (Dey et al. 1998). The calculations are performed for both normal and supramassive evolutionary sequences of strange stars rotating at all possible rates.